1. Field of the Invention
The present invention relates to a circuit for automatically adjusting gain variations resulting from a fabrication process for an amplifier to be mounted on an integrated circuit (hereinafter abbreviated as xe2x80x9cICxe2x80x9d) and to an amplifier to which the automatic gain adjustment circuit is applied. In particular, it relates to an amplifier effective in a low-noise amplifier (hereinafter abbreviated as xe2x80x9cLNAxe2x80x9d) for use in a communication transceiver or the like.
2. Description of the Related Art
As shown in FIG. 18, an LNA 182 is used in a mobile terminal such as a communication transceiver to amplify an extremely small signal received by an antenna 181. The received signal amplified by the LNA 182 is subjected to frequency conversion in a mixer 183, supplied to a programmable gain amplifier 185 through a band pass filter 184, and subsequently transmitted to a demodulation circuit. A small signal amplifier such as the LNA 182 is normally formed as an IC. In that case, proper setting of the operating points (operating current and operating voltage) and gain of an amplifying element and the stabilization thereof becomes important.
FIG. 19 shows a typical conventional example of an amplifier formed as an IC having a bias circuit for adjusting the operating points. A source-grounded MOS (Metal Oxide Semiconductor) transistor (hereinafter abbreviated as xe2x80x9cMOSTxe2x80x9d) 21 is used for the amplifier. A signal inputted from an input signal source 1 to an input terminal 17 is amplified and an output signal is retrieved from an output terminal 16 formed at the connection point between the drain of the MOST 21 and a load (ZL) 12. The operating current Id of the MOST 21 is adjusted by a bias circuit 14 via a resistor 15. The adjustment involves adjusting a DC output voltage at the output terminal 16 such that the dynamic range of an ac output signal is ensured or adjusting an output current flowing in the MOST 21 such that the transconductance of the MOST 21 and the gain determined by the load 12 have respective design values.
On the other hand, there has been known a circuit shown in FIG. 20 as means for implementing a method for independently controlling the gain without changing the voltage at the output terminal (see, e.g., JP-A No. 308651/2001). In the drawing, a current in a constant current source 205 connected in parallel with a load 204 is adjusted such that a current allowed to flow in a MOST 201 has a specified value, i.e., that the transconductance (hereinafter abbreviated as xe2x80x9cgmxe2x80x9d) of the MOST 201 has a specified value, whereby the gain of the MOST 201 is controlled to a desired value. At this time, a bias voltage is supplied to the MOST 201 via a resistor 206 but a voltage EO at an output terminal 16 hardly changes since it is determined by the gate-to-source voltage VGS in the MOST 201. The operating current of a transistor 203 is determined by a current source 202.
However, the amplifier shown in FIG. 19 suffers power supply fluctuations and temperature fluctuations in an actual situation and further undergoes device variations resulting from changes in fabrication conditions even if it has been designed optimally at a given power supply voltage, at a given temperature, and under given manufacturing conditions. Therefore, the operational voltage, i.e., the DC output voltage at the output terminal 16 and the gain are mostly different from design values. In addition, the gain of the amplifier is normally designed to prevent the distortion of an ac output waveform even if such power supply fluctuations, temperature fluctuations, and device variations as to satisfy conditions under which the gain becomes maximum, i.e., maximum gain conditions are encountered. This has caused the problems that, if such power supply fluctuations, temperature fluctuations, and device variations as to satisfy the minimum gain conditions are encountered, an output signal becomes smaller and the DC output voltage at the output terminal 16 greatly changes simultaneously. A change in DC output voltage affects an input bias voltage in an amplifier in the subsequent stage.
In the amplifier shown in FIG. 20, a current value in the constant current source 205 is fixed when the amplifier is formed as an IC so that it is impossible to tolerate power supply fluctuations, temperature fluctuations, and device variations. Since the voltage EO is determined by the voltage VGS, as stated previously, the voltage EO cannot be controlled to an arbitrary value.
It is therefore an object of the present invention to solve the problems of the conventional amplifiers described above and provide an automatic gain adjustment circuit for automatically adjusting the gain and DC output voltage of an amplifier against power supply fluctuations, temperature fluctuations, and process variations and an amplifier using the automatic gain adjustment circuit.
In accordance with the present invention, feed back control is performed with respect to a bias circuit for gain adjustment and a variable current source for DC output voltage adjustment, each of which is provided in the automatic gain adjustment circuit, so that the gain and DC output voltage of an amplifier are set automatically. This allows the set gain and DC output voltage to be held constant even if a power supply and a temperature fluctuate after the fabrication of the amplifier as an IC or if the IC fabrication process varies.
To solve the problems, the present invention uses a method illustrated in FIG. 1 as a gain adjustment method. In the drawing, the adjustment is performed in a bias circuit 14 and a variable current source 13.
An amplifying element 11 having a control electrode a, a ground electrode b, and an output electrode c is composed of, e.g., a MOST, a bipolar transistor, a metal semiconductor (MES) transistor, a hetero-junction transistor, or the like. A bias voltage is supplied from the bias circuit 14 to the control electrode a of the amplifying element 11 via a resistor 15, which determines the operating current of the amplifying element 11. A signal to be amplified is inputted from the input signal source 1 to the input electrode 17 by the control electrode a. The load 12 is connected between the output electrode c of the amplifying element 11 and a power supply Vdd, while the variable current source 13 is connected to the output electrode c of the amplifying element 11. The output terminal 16 is disposed at the connection point between the load 12 and the output electrode c of the amplifying element 11.
The operating current of the amplifying element 11 is the sum of the current in the variable current source 13 and a current flowing in the load 12. A voltage at the output terminal 16 is determined by the load 12 and a current flowing in the load 12.
If a bias voltage in the bias circuit 14 is changed while a current in the variable current source 13 is held constant, the operating current of the amplifying element 11 changes and the current flowing in the load 12 change so that a DC output voltage at the output terminal 16 changes.
If the current in the variable current source 13 is changed while the bias voltage in the bias circuit 14 is held constant, the operating current of the amplifying element 1 hardly changes, while the current flowing in the load 12 changes, so that the DC output voltage at the output terminal 16 changes. The reason for the operating current of the amplifying element 1 which hardly changes even if the DC output voltage changes is that the internal impedance of the amplifying element 11 is generally extremely high and hence the amplifying element 11 can be regarded as a substantially constant current source.
By adjusting the current value in the variable current source 13 and the bias voltage in the bias circuit 14, therefore, it becomes possible to change the operating current without changing the DC output voltage at the output terminal, conversely change the DC output voltage at the output terminal without changing the operating current, or simultaneously change the operating current and the DC output voltage at the output terminal. It is to be noted that the bias voltage in the bias circuit 14 is supplied via the resistor 15.
The signal inputted from the ac input signal source 1 to the input terminal 17 is amplified by the amplifying element 1. As represented by the numerical expression (1) the gain G of the amplifying element 11 is expressed approximately as the product of the transconductance gm of the amplifying element 11 and the load (ZL) 12:
G≈gmxc2x7ZL . . . xe2x80x83xe2x80x83(1).
If the amplifying element 11 is composed of, e.g., a MOST, on the other hand, the transconductance of the amplifying element 11 and the operating current Id thereof has a relationship represented by the following numerical expression (2) therebetween:
gmxe2x88x9d(Id)1/2. . . xe2x80x83xe2x80x83(2).
If the amplifying element 11 is composed of a bipolar transistor, the transconductance of the amplifying element 11 and the operating current Ic thereof has a relationship represented by the following numerical expression (3) therebetween:
xe2x80x83gmxe2x88x9d(Ic). . . xe2x80x83xe2x80x83(3).
By adjusting the currents Id and IC in the amplifying element by controlling the bias voltage in the bias circuit 14 and adjusting the DC output voltage by controlling the current allowed to flow in the variable current source 13, therefore, a specified gain and a specified DC output voltage can be obtained at the same time.
Although each of the following embodiments will describe the case where a MOST is used as an amplifying element, the present invention is also applicable to the case where another type of semiconductor amplifying element is used and achieves the same effects.